Generally, the heavy petroleum oil having an API specific gravity of not more than 25 contains in a large proportion a heavy fraction called "asphaltene" which has a molecular weight of not less than 1000, a heavy aromatic content and a high fixed carbon content. This heavy petroleum oil, therefore, has an extremely high specific gravity and viscosity and a high ash content and accordingly, is difficult to handle. When the heavy petroleum oil is used as a fuel, for example, it usually has its viscosity lowered by addition of a light fraction, although it may be used in its unmodified form in some cases. When a blend of reduced viscosity is used as a fuel in a boiler for example, it may cause coking or scale formation in the boiler (which problems are held to originate in the asphaltene contained in the blend). Thus, the blend is not suitable as a fuel.
For the heavy petroleum oil to be advantageously used as a fuel or for industrial purposes, it has been conventional practice to thermally crack the heavy petroleum oil within a tubular type heating furnace for conversion into a light oil. In the course of this thermal cracking treatment, however, coke is produced from the feed oil and is suffered to deposit inside the furnace tube and the deposited coke produces a clogged furnace interior and degraded thermal conductivity, with the result that the furnace operation is jeopardized and the resultant light oil is adversely affected in terms of quality and yield. Moreover, whenever there occurs deposition of coke and other residues in the furnace interior, it becomes necessary for the furnace interior to be free from the deposited coke. The operation for the removal of the deposited coke requires much time and labor and involves harsh conditions and, consequently, gradually aggravates the wear of the furnace.